1. Field of the Invention
The present invention relates to synthetic-antenna side sonars which make it possible to obtain a sonar image by scanning through water abeam of a boat which is carrying or towing the sonar, and by using the echoes of the signals transmitted by a transmission antenna such as the antenna 101 in FIG. 1 for a plurality of successive positions of the boat.
2. Discussion of the Background
Synthetic-antennas are known in which a virtual antenna is formed from the N different spatial positions of a physical antenna such as the antenna 102 in FIG. 1. The role of the physical antenna is, on the one hand, to ensure sufficient rejection of the array lobes of the physical antenna by placing them in the directionality zeros of the physical antenna (first condition) and, on the other hand, to ensure a sufficient directionality gain in the direction of the focal point for N successive integrated recurrences (second condition).
It will be noted that the antenna 101 may or may not form part of the physical antenna 102.
The first condition requires an angular directionality of the physical antenna at least equal to the angular period of the array lobes. This is equal to ##EQU1##
where T is the inter-recurrence period, .lambda. the transmission wavelength and v the speed of the boat carrying the sonar. This leads to the well-known sampling condition: ##EQU2##
Since the inter-recurrence period T is fixed by the maximum range of the sonar, (1) limits the product of the speed times the maximum range, that is to say the hourly coverage of the synthetic-antenna sonar.
The second condition fixes the maximum value of N, and therefore the resolution of the synthetic-antenna sonar.
Thus, in the simple case in which the directionalities on transmission and on reception for the physical antenna are identical and equal to: ##EQU3##
with u=sin.theta. where .theta. is the bearing and with the definition ##EQU4## PA1 the length of the synthetic antenna is limited to: ##EQU5## PA1 where .rho. is the range of the sonar. PA1 and consequently the side resolution to ##EQU8##
The side resolution is then given by the conventional formula: ##EQU6##
When the physical antenna is itself an array, which is very frequent in sonar, the directionalities on transmission and on reception are no longer necessarily identical.
It is, for example, possible to broaden the transmission sector by transmitting with a restricted portion 101, of length L.sub.e, of the physical antenna 102, and to form a set of physical reception channels in this sector.
When the synthetic antenna is formed with a physical reception channel of fixed aiming direction, cutting out a given subsector of this broadened transmission sector, a so-called "isingle beam" synthetic-antenna sonar is obtained whose properties are essentially equivalent to the simple synthetic antenna described above, for which the directionalities on transmission and on reception are equal. In particular, its side resolution is again given by (4).
By cutting the transmission sector up into different subsectors corresponding to a set of physical reception channels, it is possible to form simultaneously a plurality of single-beam synthetic antennas which can then be integrated incoherently. A so-called "incoherent multibeam" synthetic antenna is then obtained. This antenna has the same side resolution as the single-beam synthetic antennas of which it is formed, but the incoherent addition makes it possible to reduce the variance of the reverberation and to improve the radiometric resolution of the image.
Coherent addition of these single-beam synthetic antennas leads to the so-called "coherent multibeam" synthetic antenna, which makes it possible to increase the length of the synthetic antenna to: ##EQU7##
All these operating modes, which are well known, make it possible to obtain improved images compared with the simple mode, but in no way alter the first constraint (1) which must still be satisfied. However, this constraint is very severe in practice, in particular for medium-range bed imaging systems (typically 3000 m for the TOBI system from SOC [Southampton Oceanographic Centre, United Kingdom]) for which it might be desired to reduce the side resolution (this is about 50 m at 3000 m for TOBI) by synthetic aperture. Thus, for physical antenna lengths of respectively L=3 m, L=4 m or L=5 m, this first constraint limits the maximum range to only 1125 m, 1500 m or 1800 m, even when operating at the slow speed of v=1 m/s typical of towed sonars in deep water.
It is therefore desirable to be able to increase, for example to double, the maximum range of the synthetic antenna in comparison with that given by (1), even at the cost of increasing, for example doubling, the side resolution compared with (4). With an antenna such that L=4 m, bed imaging systems ranging to 3000 m with a side resolution of 4 m could be obtained. In the prior art as it now stands, it is known that the side resolution for L=4 m is 2 m for the simple operating mode and may be reduced further, if so desired, by employing the coherent multibeam mode, but extending the range up to 3000 m requires a physical antenna of length L=8 m regardless of the mode adopted, which is impossible to reconcile with normal bulk constraints.
It is also desirable to be able to increase the speed for a given range.
A sonar system known by the name of interferometric transmission sonar is furthermore known [1] from a French Patent No. 2 412 177 filed by the company THOMSON-CSF, having G. GRALL as its inventor, in which acoustic signals are simultaneously transmitted at the two ends of the reception antenna. However, this sonar system is not used to form a synthetic antenna.
U.S. Pat. No. 5,295,118 [2] granted on Mar. 15, 1994 to Westinghouse Elec. Corp. under the rights of G. A. GILMOUR is also known, which describes a sonar which itself also uses two transmitters lying at the two ends of the antenna, but transmitting alternately (alternation on each recurrence) in two distinct spectral bands. According to the author, alternate transmission makes it possible to double the hourly coverage compared with (1) without loss of performance. In fact, this is not at all true. Each spectral component of the synthetic antenna is undersampled by a factor of two, and the same is true for the total synthetic antenna. The loss of performance is therefore identical to that of the conventional method, with fixed transmission throughout the spectral band, when (1) is no longer satisfied.